Formation of the χ-Phase Precipitate in Co-28Cr-6Mo Alloys with Additional Si and C

Abstract

The precipitates of biomedical Co-28Cr-6Mo-(0.5 to 2)Si-(0.05 to 0.35)C alloys (mass pct) have been investigated before and after heat treatment, focusing on the formation of the χ-phase precipitate. The precipitates were precisely and directly analyzed after using electrolytic extraction to separate the precipitates from the alloys. Heat treatment was performed at 1523 K (1250 °C) for a holding time of 0.6 ks. The χ-phase precipitate was detected with Si content of 1.3 to 2 mass pct and C content of 0.05 to 0.15 mass pct in both the as-cast and heat-treated alloys. The higher Si content and the mid-level C content of about 0.15 mass pct favored the formation of the χ-phase precipitate. Moreover, the χ-phase precipitate was not observed in the compositional range of the ASTM F 75 standard: Si content ≤1.0 mass pct and C content ≤0.35 mass pct. In the as-cast Co-28Cr-6Mo-1.3Si-0.15C alloy, which is outside of the ASTM F 75 standard, the content of the χ-phase precipitate was around 6 pct in area percent; the decrease in the ductility was detected in the tensile test of this alloy. The amount of precipitates decreased due to heat treatment at 1523 K (1250 °C), where the dissolution of precipitates occurred. After the heat treatment, a single χ-phase precipitate region was detected.